1. Field of the Invention
The present invention relates to a linear image sensor, and more particularly to a linear image sensor that is capable of reducing a layout area.
2. Description of the Related Art
As a conventional technique, JP 07-226495 A discloses a linear image sensor in which a shutter structure is provided between photodiode arrays. FIG. 1 shows an example of an overall construction of such a linear image sensor having an electric shutter structure between photodiode arrays. FIG. 2 is a cross-sectional view taken along the line C-C′ of FIG. 1 and is a channel potential diagram thereof.
In this linear image sensor, electric charges photoelectrically converted by and accumulated in each photodiode of photodiode arrays 101a and 101b are read by readout gates 102a and 102b into CCD shift registers 103a and 103b that are respectively adjacent to the readout gates 102a and 102b. The electric charges read into the CCD shift registers 103a and 103b are sequentially transferred by the CCD shift registers 103a and 103b that perform two-phase (φ1, φ2) driving and are outputted to the outside by output circuits 104a and 104b. The output circuits 104a and 104b are each formed of a signal charge detection unit and an analog circuit such as a source follower circuit or an inverter. Here, the signal charge detection unit is formed of a floating diffusion region and converts signal charges into a signal voltage.
On the other hand, in the linear image sensor, shutter gates 105a and 105b are positioned on a side opposite to the readout gates 102a and 102b with the photodiode arrays 101a and 101b in-between. The shutter gates 105a and 105b discharge the electric charges photoelectrically converted by and accumulated in each photodiode of the photodiode arrays 101a and 101b to a shutter drain 106, thereby performing adjustment of an exposure time period.
As shown in FIG. 2, this linear image sensor has a structure where the photodiode arrays 101a and 101b that are each formed of an N-type region 111 and a P-type region 112, the readout gates 102a and 102b and the shutter gates 105a and 105b that are each formed of a polycrystalline silicon electrode 114b, the CCD shift registers 103a and 103b that are each formed of a polycrystalline silicon electrode 114a and an N-type region 110, and the shutter drain 106 formed of an N-type region 115 are provided on a P-well 108 formed on an N-type silicon substrate 107.
Also, in FIG. 2, reference numeral 113 denotes a thermal oxidation film and reference numeral 116 indicates an interlayer insulating film. Here, a metallic wiring made of aluminum or the like for supplying an input clock signal to each of the polycrystalline silicon electrodes 114a and 114b is omitted because the metallic wiring is not required for explanation of the present invention.
When a color linear image sensor is constructed, three linear image sensors having the construction described above are provided parallel to each other on the N-type silicon substrate 107 and color filters having different colors (green, blue, red) are respectively overlaid on the photodiode arrays 101a and 101b of the three linear image sensors. In this case, it is required to adjust an exposure amount (product of the amount of light incident on the photodiodes and an accumulation time period) for each color.
One of characteristics that determine the performance of an image sensor is a saturation output voltage. In general, the output signal voltage of an image sensor is proportional to an exposure amount (product of the amount of light incident on a light-receiving portion and an accumulation time period). Once the exposure amount exceeds a certain value, however, the output signal voltage becomes incapable of increasing any more. This output signal voltage incapable of increasing any more is called a “saturation output voltage”. Also, an exposure amount giving the saturation output voltage is called a “saturation exposure amount”. As this value increases, a usable signal voltage amplitude is increased and a dynamic range (ratio between the saturation exposure amount and a noise such as a dark output) is also increased. As a result, it is required to increase the saturation output voltage as much as possible from the viewpoint of the performance of the image sensor.
In the case of the color linear image sensor described above, the saturation output voltages of the three linear image sensors provided with the color filters become the same unless the sizes of the photodiodes or the CCD shift registers are intentionally changed or the maximum signal voltage amplitudes are intentionally changed in respective output circuits.
Also, as described above, it is preferable that the saturation output voltages are increased as much as possible from the viewpoint of the performance of the color image sensor. Therefore, it is natural that the same saturation output voltage is set for the three colors. In the case of the color linear image sensor described above, however, the output sensitivities (output signal voltages/exposure amounts) of RGB outputs are usually not the same. Further, even if the RGB outputs have the same sensitivity under a certain light source, when the light source to be used is changed, differences may occur in sensitivity among the RGB outputs. Accordingly, in general, relationships shown in FIG. 3 exist between the exposure amounts and the signal output voltages in the color linear image sensor. FIG. 3 shows a case where among the RGB outputs, the green output has the maximum sensitivity and the blue output has the minimum sensitivity.
As can be seen from FIG. 3, although the RGB outputs originally have the same saturation output voltage Vsat, it is impossible to increase output values of the red and blue outputs above VsaR and VsaB, respectively, with the green output having the maximum sensitivity. This is because when this color linear image sensor is used while exceeding a saturation exposure amount SEG (exposure amount giving the saturation output voltage of the green output), the green output exceeds the saturation output voltage and it becomes impossible to obtain normal green image data. Also, there is a fear in that signal charges may overflow from the photodiode portion or the CCD shift register related to the green output and flow to other photodiode portions or CCD shift registers related to the remaining two colors, thereby causing color mixture. In either case, in this example, the green output has the maximum substantial saturation output voltage and the blue output has the minimum substantial saturation output voltage, which leads to a situation where differences occur in dynamic range among respective colors and an influence is exerted on image quality.
In view of this problem, a shutter structure has conventionally been used, as described above. With this construction, it becomes possible to independently control the accumulation time periods for the RGB colors while setting the same light amount for these colors. A driving method used in this case is shown in a timing chart of FIG. 4. By independently adjusting driving pulses applied to the shutter gates for respective colors, an optimum exposure amount is obtained for each color and it becomes possible to use the linear image sensor until a saturation output voltage common to the three colors is reached.
When a linear image sensor having two photodiode arrays is provided with the shutter structure described in the above conventional example between the photodiodes in order to improve MTF between the photodiodes or the like, there is a case where a distance between the two photodiode arrays becomes a problem.
In a scanner or a copying machine using a linear image sensor, the linear image sensor is mechanically scanned in an auxiliary scanning direction that is perpendicular to a main scanning direction in which photodiode arrays are provided. In order to obtain image information of a predetermined area of a subject, after the first photodiode array finishes scanning the predetermined area of the subject, it is necessary to externally store information until the second photodiode array finishes scanning the predetermined area and to perform alignment and signal processing on the information. As a result, it is necessary to use an external memory.
In a linear image sensor of 10600-pixel class that is applied to a high-resolution scanner or copying machine, for instance, when a gray scale (gradation between black and white) is expressed using 12 bits, the capacity of a memory required becomes as follows.C=10600×12×(M+1)bits  (1)where, M is a value obtained by expressing the interline distance between the two photodiode arrays using the number of scanning operations.
When the size of each photodiode is 4 μm×4 μm and the interline distance between the photodiode arrays is 12 μm, for instance, M becomes as follows.M=12 μm/4 μm=3  (2)As a result, the external memory is required to have a capacity of 508800 bits. As can be seen from Equation (1) described above, in order to reduce the capacity of the external memory, it is necessary to shorten the interline distance between the photodiode arrays, thereby reducing the number of scanning operations performed between the scanning by the first photodiode array and the scanning by the second photodiode array.
Further, mechanical scanning is performed in the auxiliary scanning direction, so that there also exists a phenomenon called “color drift”. Assuming that scanning drift Y having the same amount occurs each time a scanning operation is performed, for instance, the total amount of drift occurring between the scanning by the first photodiode array and the scanning by the second photodiode array becomes as follows.YA=M×Y  (3)As can be seen from this Equation (3), in order to reduce the color drift, it is necessary to shorten the interline distance, thereby reducing the number of scanning operations performed between the scanning by the first photodiode array and the scanning by the second photodiode array.
Further, as can be seen from FIG. 2, factors generating this interline distance M are the total width of two polycrystalline silicon electrodes constituting the shutter gates and the N-type region constituting the shutter drain.
When a color linear image sensor is produced using this linear image sensor, it is apparent due to the same reason as above that when interline distances among respective colors are shortened, the number of scanning operations performed between the scanning for the first color and the scanning for the third color is reduced. As a result, it becomes apparently possible to reduce a capacity of a memory used and to suppress an influence of the color drift.